Detection of frequency modulated signals



July 14, 1931.

c. w. HANsELL DETECTION OF FREQUENCY MODULATED SIGNALS Filed Jan. 30. 1929 lllllxlul 25 slope of the sides o "Patented July 14,1931

- CLARENCE w. nAn'siiLn'or roRT mFERsoii,

RAnro coRroRA'rIoisiV or AMERICA,

Loire NEW YoRx, AssIeNoR To A coRroRArroN or DELAWARE i DIRECTION OF FREQUENCY HODUI'ATEID.BIG'N'ALS l Application mea January ao, 199s. semi in. ssaioa This invention relates to a method and means for securing a response de endent upon lthe frequency of alternating e ectrical energy, and more particularly to a method and means for detecting frequency modulated signals. The primar object of my invention is `to E provide a met od and means for securing a requency indication, and to this end, I so limit the amplitude and energy per cycle of the alternating energy that the energy per unit of time is pro orti onal to the number of c cles per unit o time, and I then rectify t e resultin energy, so that the magnitude l5 of the recti ed energy may be used as an indication of the frequency of the alternating energy. n

y More specifically, I limit the amplitude of the received alternating energy so greatly that the output current is practically square in wave form, and thereafter transfer the ener through an inductive transformer, so thatilie output in the secondary of the transformer depends u on 'o the magnitude and the current wave fed to the rimary of the transformer. These sides, having already been made equal in amplitude, and of substantially the same nearly perpendicular slope, the output impulses for each cycle are the same, and the number of these impulses in a unit of timedepends solely upon the frequency of the in ut ener In order to detect frequency m ulated signalsit has heretofore been proposed to first clinge the frequency modulation to amplitude modulation by impressing the received signals on a resonant circuit tuned to a fre-Y quency lying outside the operating range of frequency, vand thereafter to detect the re. sulting amplitude modulated signals in order to.4 obtain signal frequency energy for use ina translating device. This method is diicult to free from distortion, and is'limited in usefulness by the fact that the resonance characteristic of a tuned circuitmay differ under dynamic conditions, such as exist when the frequency of the energy applied thereto is rapidly varying, `from what it is under static conditions, when each frequency tector and t 2, energy collected applied theretol is kept siderable period of time.

It is a further object of my invention to provide a frequency modulation detector which is sensitive and free from distortion. For this purposeI 'employ the method already described for obtaining frequency ind1cation,and uti1ize the signal frequency component of the rectified energy for translation.

If it is desiredto obtain linear amplitude response, instead ,of linear energy res nse, that is, if it is desired to obtain amplitudes constant for` a conproportionall to the frequency-of the received I may employ between the dee translating device an amplifier having a characteristic following a three halves or a s uare law, which results in the signal energy,

y desired trans ormation. I do not consider this a detector stage, even thou han asymmetric characteristic is employe because the 'output frequency which is actually. utilized is equal to the input frequency, that is, it is the modulation frequency.

The sensitivity of the circuit is enhanced if the percentage frequency modulation is increase and according to my invention the relatively slight vmodulation of the received carrier is increased by heterodynin the received energy with locally generate energy of constant frequency. The resulting beat energy has a variation in fre uency equal to that of the'received carrier, (but, the modulation being impressed on alower carrier, the percentage modulation is greatly increased.

My invention is further described in the following specification, which is accompanied by drawingsin-which Figure 1r is a wiring dia in for a circuit embodying the invention; igure 2 is explanatory of the operation of the invention; Fi re 3 isa portion of Figure 1 showing a different indicatin means. o l

eferring to Figure 1 of the drawings it willbe seen that t ere is an antenna system by which is fed to an amplifier 4, the output from which is preferably heterodynd in aA heterodyne detector 6 with energy ofconstant frequency obtained froina local oscillator 8, the frequency constancy of which is'maintained by a crystal 10. The energy from the heterodyne detector` 6 is frequency modulated, like the received 5 energy, but the percentage frequency modulation is greatly increased. This energy may be further amplified in an amplifier 12, which is followed by a limiter 14. The limiter is simply an amplifier adjusted t0 give anode current having a practically square wave form. Of course, the amplifier 12 may be made to act as a limiter, instead of employing a separate amplifier and limiter. The energy from the limiter 14 is fed to a rectifier 16, here shown as a full wave rectifier, through an inductive transformer 18. The transformer serves to apply to the rectifier impulses of potential the ma itude of which depends upon the amplitu e and slope of the sides of the currentwaye fed to the primary of the transformer. The anode current having been made of constant amplitude and ofy practically square wave form, these impulses become uniform, and their frequency depends upon the frequency of the received signalling energy. 1 c

This may better be explained in connection with Figure 2, showing a relatively high frequency small amplitude current waye 32, contrasted with a slower frequency higher amplitude current wave 34. Because of the limiting action the extremes of the current wave 34 are chopped off, as is indicated on the drawings. Now the potential across the seondary change of current, and therefore is not affected by the magnitude of the flat tops in the current wave, but only by the sides of the current wave, as for example the side 36 in the wave 32, and the side 38, in the wave 34, and these sides are substantially alike. It should be kept in mind that the case illustrated in Figure 2 is most unfavorable to the operation of the invention, for actually the limitingmay be adjusted, say, at the level of the dotted line 40, in which case the sides of the current wave are practically perpendicular.

The output from -the rectifier is applied to a condenser 20, which is a by-pass condenser for the radio frequency component of the rectified energy, but is of relatively high reactance to the modulation frequency. The modulation potential is applied to an amplifier tube 22, and thence to a translating device 24 of any appropriate type. The rectified output may be applied directly to the translating device, instead. of being intermediately amplified, but amplification is desir-l able, not only to strengthen the modulation energy, but also because the amplifier may be made to produce a linear amplitude response,

instead of a linear energy response, by using a tube having a characteristic which approximates a three halves or possibly even a square 55 law. I consider the tube 22 an amplifier tube,

of the transformer depends upon the rate of there is an indicating instrument 54. The

rectifier has here been illustrated as a half wave rectifier, inasmuch'as a full wave rectifier, while desirable, is not essential.

In appropriate cases the translating means may be a loud-speaker, tape recorder, picture or facsimile recorder, and so on. Also,

the signal energy may be amplified, after rectification, to any desired extent.

I claim:

1; The method of securing a response dependent upon the frequency of alternating electrical energy which includes so limiting the amplitude and energy per cycle of the energy that the energ per unit of time is proportional to the num er of cycles per unit of time, rectifying the resulting energy, and utilizing the magnitude of the rectied energy as an indication of frequency. l

2. The method of detectin frequency modulated signals which inclu es limiting the received energy, inductively transforming the limited energy, rectifyin the transformed energy, and utilizing the signal frequency component of the rectified'energy for translation.

3. The method of detecting frequency modulated signal energy which includes amplifying and so limiting the amplitude and energy per cycle of the received energy that the energy per unit of time is proportional to the number of cycles per unit of time, rectifying the resulting energy, and utilizing the rectified energy for translation.

4. The method of detecting frequency modulated signal energy which includes limiting the ,received energy, inductively transforming the limited energy, so that the energy per unit of time is proportional to the frequency, rectifyin the resulting energy, and utilizing the recti ed energyfor translation.

5. The method of obtaining distortionless I vthe signal energy, heterodyning the coll energy with locally generated energy of constant frequency, so limiting the amplitude and energy per cycle of the resulting energy that the energy per unit of time is proportional to the frequency, rectifying the resulting energy, and utilizing the rectified energy for translation.

7. The method of receiving frequency modulated signals which includes collecting the signal energy, heterodyning the collected energy with locally generated energy of constant frequency in order to increase the peroentage frequency modulation, limiting the resulting energy, inductively transforming the limited energy so that the energy per unit of time is proportional to the frequency, rectifying the transformed energy, and utilizing the vrectified energy for translation.

8. An arrangement for securing a response 1 zo dependent upon the frequency of alternating electrical energy supplied thereto comprising means to so limit the amplitude and l energy per cycle of the supplied energy that the energy er unit of time is proportional il -to the num r of cycles per unit of time, means to rectify the resulting energy, and means responsive to the avera magnitude of the rectified energy as an indication of frequency.

9. An arrangement for detecting frequencv modulated signals comprising means to limit the received energy, means to rectify the limited energy, an inductive transformer therebetween, and a' translating device responsive 8l' tothe signal frequency component of the rectified energy.

lO. An arrangement for detecting frequency modulated signal ener comprising means to amplify and so limit t e amplitude and energy per cycle of the received energy that the energy per unit of time is proportional to the number of cycles per unit of time, means to rectify the resultlng energy, and a translating device responsive to' the rec- 'M tified energy.

11. An arrangement for detecting frequency modulated signal energy comprising means to limit the received ener means to inductively transform the limite energy, so

that the energy per unit of time is proportional to the frequency, means to rectify the resulting energy, and means to translate the rectied'energy.

frequency modulated signals comprising means to collect the signal energy, means vto heterodyne the collected energy with locally generated energy of constant frequency, means to so limit the amplitude and ener per cycle of the resulting energy that the energy per unit of time is roportional to the frequency, means to recti y the resulting energy, and means to translate the rectified energy.

14. An arrangement for the reception .of frequency modulated signals comprising means to collect the signal energy, means to heterodyne the collected energy with locally generated energy of constant fre uency in order to increase the percentage requency modulation, means to limit the resulting ener?, means to inductively transform the lim- 'te energy so that the energy per unit of time is roportional to the frequency, means to rectif) the transformed energy, and means to trans ate the rectified energy.

CLARENCE W. HANSELL.

12. An arrangement for distortionless dei tection of frequency modulated signal energy comprising means to limit the received l energy, means toinductively transform the limited energy, so that the energy per unit of time is proportional to the frequency,

'Tdi means to rectify the resulting energy, means to amplify the rectified ener in accordance with a characteristic approximately following a three halves law, and means to translate the amplified energy. 'al 13. An arrangement for the reception of 

